Answer (d)
What is a Black Swan?
The term was popularized by Nassim Nicholas Taleb, a finance professor, writer, and former Wall Street trader.
A black swan is an unpredictable event that is beyond what is normally expected of a situation and has potentially severe consequences. Black swan events are characterized by their extreme rarity, their severe impact, and the widespread insistence they were obvious in hindsight.
Black swan events can cause catastrophic damage to an economy, and because they cannot be predicted, can only be prepared for by building robust systems.
Taleb describes a black swan as an event that 1) is beyond normal expectations that is so rare that even the possibility that it might occur is unknown, 2) has a catastrophic impact when it does occur, and 3) is explained in hindsight as if it were actually predictable.